In my own observation, this subject has something to do with there the machines are made. I don't claim to be an expert, nor do I have any inside knowledge of how TTI does their business. My experience comes from having worked on many machines made before and after TTi.
Hoover machines made in Mexico, in my opinion, are generally of a basic higher quality than those made in China. I believe Hoover still manufactures some of their legacy models there, such as the bagged self propelled Windtunnel. I've worked on these with dates as late as 2012, well after the TTI takeover. Same comment re. bagged Tempos and dual chamber bagless Windtunnels. The materials used appear to be superior, however the motors are Chinese so obviously not made in North America.
The machines sourced in China, again as a matter of my own opinion, seem to be lighter weight and plastic materials that are not as robust as those coming from Mexico. There are differences from one Chinese-made model to another. For example, the UH70000 series (Cyclonic) seems to be different enough in execution from the T Series (as another example) that they likely are made in two different plants.
And does TTI even own a factory or factories in China? Or do they just contract out to various Chinese electrical firms to make X hundred thousand machines, then move on? If they are contracting out to bidders then it makes sense that different types of plastic used, different hardware, etc. would exist from one model line to another.
Sears relied for years on Panasonic USA to supply many of their machines and the common manufacturing techniques and materials used are constant across production. However, when they sourced some machines from Phone Mate in China for a few years, you could readily see the differences in how they were made.